Field of the Invention
The invention relates to the use of dicyanamide-containing ionic liquids which accelerates the cure and decreases the cure temperature of cyanate esters to form unique thermoset polymers having a triazine network.
Brief Description of the State of the Art
The unique ring-forming trimerization cure mechanism and rigid thermoset structure of cyanate ester resins (CEs) give them a range of valuable properties for high performance materials applications. Nair, C. et al., Cyanate Ester Resins, Recent Developments. Springer Berlin/Heidelberg. 2001; Vol. 155, pp 1-99.
The trimerization of cyanate ester monomers can occur without additives, although this is thought to proceed due to the presence of trace monomer synthesis catalysts or residual phenols as taught by Chen, C.-C. et al. (A kinetic study on the autocatalytic cure reaction of a cyanate ester resin, Journal of Applied Polymer Science 2004, 92 (5), 3067-3079). Typically, active hydroxyl donors such as nonylphenol in concert with metal catalysts such as titanium, cobalt, copper, chromium, iron, tin, zinc, or manganese are used to accelerate cure speed and extent. A simplified scheme of trimerization is shown in FIG. 1 (excluding side reactions), involving a cyclotrimerization reaction of a cyanate ester monomer wherein both catalysis mechanisms, one catalyzed by a hydroxyl source and the other by a metal catalyst are given. Initially, the hydroxyl source combines with a cyanate ester monomer having structure (1) to form an imidocarbonate having structure (2). The metal-stabilized imidocarbonate can interact with a second monomer to form a stabilized dimer having structure (3). A third monomer completes the triazine ring of structure (4), and the hydroxyl source and metal catalyst are recycled for further reaction.
These catalysis systems have several downsides. Nonylphenol acts as an endocrine mimic, with metabolic and reproductive consequences in many aquatic species, with feminization of males, sterility, and juvenile death occurring with concentrations as low as 8.2 μg/L, as discussed in Soares, A. et al. (Nonylphenol in the environment: A critical review on occurrence, fate, toxicity and treatment in wastewaters. Environment International 2008, 34 (7), 1033-1049); and Giger, W. et al. (4-Nonylphenol in sewage sludge: accumulation of toxic metabolites from nonionic surfactants. Science 1984, 225 (4662), 623-625). Metallic coordination catalysts, particularly copper, have been shown to accelerate hydrolysis within cured cyanate ester resins, leading to significant network degradation, Tg loss, and blistering, see Hong, S.-G. et al. (Catalytic effects of copper oxides on the curing and degradation reactions of cyanate ester resin. Journal of Applied Polymer Science 2007, 104 (1), 442-448); Zacharia, R. E. et al. (Effect of catalyst on the thermal degradation of a polycyanurate thermosetting system. Journal of Applied Polymer Science 1997, 64 (1), 127-131); and Marella, V. V., et al. (Hydrolytic degradation of highly crosslinked polyaromatic cyanate ester resins. Polymer Degradation and Stability 2014, 104 (0), 104-111).
Accordingly, it is an object of certain embodiments of the present invention to provide a curable composition which can be cured to form a thermoset polymer having a triazine network, wherein the thermoset polymer has a range of valuable properties for high performance materials applications.
Another object of certain embodiments of the present invention is to incorporate ionic liquids comprising dicyanamide anions that accelerate the cure of the curable composition to form the thermoset polymer having a triazine network.
Another object of certain embodiments of the present invention is to use a synthesis strategy which modifies the network structure and removes or reduces the need for metal catalysts which contribute to cyanate ester degradation, potentially decreasing the rate of hydrolytic degradation and its associated Tg loss.